IJCTA Vol.8, No.1, Jan-June 2015, Pp.336-344 © International Sciences Press, India

Mitigation of Subsynchronous Resonance Oscillations Using Static Synchronous Series Compensator Mr.J.Sreeranganayakulu1, Dr.G.V.Marutheswar2 and Prof.K.S.R.Anjaneyulu3 1 Assistant Professor, Dept. of EEE, SITAMS, Chittoor, A.P., INDIA 2 Professor, Dept. of EEE, Sri Venkateswara University, Tirupati, A.P., INDIA 3 Principal, JNTU College of Engineering, JNTUA, Anantapuramu, A.P., INDIA Email:-: ranga2k6@gmail.com ABSTRACT

Subsynchronous Resonance (SSR) is a serious problem due to series capacitor placement in power system networks which causes uncontrolled oscillations in the mechanical system of a multi mass model [1]. Many models have been developed to mitigate these oscillations which generally occur during a three phase fault on the power system. In this paper IEEE second benchmark system [2] is investigated using MATLAB based on the Power System Block-set (PSB) in conjunction with Simulink. Here, this model is simulated without FACTS controllers and then the Static Synchronous Series Compensator (SSSC) is included in the model and the results are compared for their effectiveness in damping the SSR oscillations. Keywords: FACTS, Sub synchronous Resonance, SSSC, Torsional Oscillations.

1. INTRODUCTION

Worldwide series capacitors have been extensively used for improving power transmission in long distance transmission lines. While it has been known that series capacitors can cause self excited oscillations at low frequencies (due to low X/R ratio) or at Sub synchronous frequencies (due to induction generator effect), the problem of self excited torsional frequency oscillations (due to torsional interactions) was first experienced at Mohave power station in U.S.A. in December 1970 and October 1971. The problem of self excitation due to torsional interaction is a serious problem and led to detailed analysis and study [5].

Subsynchronous resonance is electric power system condition where the electric network exchanges energy with a turbine generator at one or more of the natural frequencies of the combined system below the synchronous frequency of the system [1].

While steady state SSR problem is conveniently studied by the characterization of operating point stability based on small signal analysis, the study of the transient torques requires the consideration of nonlinear models. This is due to the fact that transient torques which can damage shafts arise from large disturbances such as faults and reinsertion of series capacitors.

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SSSC is shown simplified in Figures 6 & 7 together with the relative voltage phasor diagram. The phasor diagram clearly shows that at a given line current the voltage across the series capacitor forces the opposite polarity voltage across the series line reactance to increase by the magnitude of the capacitor voltage.

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342

Table 1. Peak Torques and speed deviations of the two cases

4. CONCLUSIONS

This paper illustrates the effectiveness of SSSC in damping the sub synchronous resonance oscillations observed in large multi mass model consisting various turbines, exciter and synchronous generator. Here, two cases are investigated i.e. without and with the inclusion of SSSC. These results clearly show that the without SSSC the oscillations in the mechanical system are between -2.4pu and 2.5 pu between generator-LP turbines and between -1.1pu and 1.15 between LP-HP turbines. With the inclusion of SSSC the oscillations were can reduced to -0.89pu and 0.97 between the turbine generator and -0.45 and 0.54 between LP-HP turbines. Also the change in angular velocity is also reduced which can be observed from graph and table 1. Hence the performance of the network is improved due to inclusion of SSSC.

Appendix

AC System Parameters:

System Voltage: 500 kV

System frequency: 50 Hz

1. Parameters of both the Transmission lines

(3 phase):

Resistance/km : 0.02 ohm,

Inductance/km : 0.9 H

Capacitance/km: 13 µF,

Case

Gen-LP Torque (p.u.)

LP-HP Torque (p.u.)

dω (Gen) p.u.

dω (LP) pu

dω (HP) pu

Min

Max

Min

Max

Min

Max

Min

Max

Min

Max

Without SSSC

-2.4 2.5 -1.1 1.15 -0.012 0.0097 -0.0072 0.0058 -0.0145 0.0145

With SSSC

-0.89 0.97 -0.45 0.54 -0.006 0.008 -0.0025 0.0034 -0.0061 0.0062

343

Length of Transmission Line: 200 km Capacitive reactance: 30% of inductive reactance.

2. Data of Synchronous Generator ( 3 phase, round rotor ):

Nominal Power: 600MVA

L-L voltage: 22 kV

Reactances: Xd = 1.65, Xd’ = 0.25, Xd’’ = 0.2, Xq = 1.59, Xq’= 0.46, Xq’’=0.2

D- axis time constant : open circuit

q- axis time constant : open circuit

Time Constant:

Tdo’= 4.5 sec,Tdo’’= 0.04 sec,

Tqo’= 0.67 sec,Tqo’’= 0.09 sec

Stator resistance= 0.0045 pu

3. Power Transformer (Delta/Star):

Nominal Power: 600 MVA( 3 phase )

Rated voltage: 22kV/500kV (Line-Line)

REFERENCES

[1] IEEE SSR working group, “proposed terms and definitions for sub synchronous resonance,” IEEE symposium on countermeasures for sub synchronous resonance, IEEE Pub.81TH0, p086-9-PWR,1981 p 92-97.

[2] IEEE SSR working group,”First benchmark model for computer simulation of sub synchronous resonance“.IEEE transactions on power apparatus and systems, vol. PAS-96, pp. 1565-1572,September/october 1977.

[3] IEEE SSR working group,”Second benchmark model for computer simulation of sub synchronous resonance “.IEEE transactions on power apparatus and systems, vol. PAS-104, pp. 1057-1066,may 1985.

[4] Dr. Narendra Kumar, Sanjiv Kumar and Vipin Jain, “Damping Sub synchronous Oscillations In power system using shunt and series connected FACTS controllers”, IEEE transactions on power delivery, vol.2, Sep 2010.

[5] P.M. Anderson, B.L. Agrawal, J.E. Van Ness, “Sub synchronous Resonance in Power Systems”, IEEE Press, New York, 1990, pp.31-93.

[6] P. Kundur, Power System Stability and Control, EPRI Power System Engineering Series, New York, McGraw-Hill Inc., 1994.

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[7] N.G. Hingorani & L. Gyugyi, ”Understanding of FACTS”, IEEE Press.

[8] J.Guo, M.L.Crow, J.Sarangapani “An improved UPFC Control for Oscillation Damping” IEEE Transactions on Power Systems, Vol. 24,No. 1, Feb. 2009,pp.288 – 296.

[9] J.Sreeranganayakulu, N.Chinna Alluraiah, “Simulation of first benchmark model for analysis of sub synchronous resonance in power systems using SEQUEL”, International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 National Conference on Recent Advances in Power and Control Engineering (RAPCE-2011), Dec 2011.